Method of treating tin



Patented Feb. 23, 1943 METHOD or TREATING TIN Willard 0. Cook and Hugh E. Ronline, Pittsburgh,

Pa., assignors to Carnegie-Illinois Steel Corporation, a corporation of New Jersey N Drawing. Application April 29, 1939,

- Serial No. 270,913

1 Claim.

The present invention relates to a method of treating tin to produce an article of manufacture whose surface characteristics are such that the tin will not darkly stain or discolor when used in the packing of certain materials.

An object of the present invention relates to subjecting tin coated material to the electrolytic action, as an anode, in an alkaline electrolyte for the purpose of oxidizing the tin coated surface so as to eliminate theunsightly discoloration generally appearing on the inner surface of tinned containers after or during the packing of certain materials which have a tendency to react with tin. v

One of the main problems confronting the tinning and canning industries today is the elimination of the dark discolorations which appear on the inner surfaces of the tinned containers during or after the processing of certain products packed therein. Such dark discolorations are particularly associated with the packing of materials containing sulphur-bearing protein matter and are, therefore, generally believed to be thin films of tin sulphide. Corned beef and similar meat commodities, vegetables such as corn, etc., are a few of the products in the packing of which such discolorations occur.

Treatment of tin plate in an aqueous solution of potassium permanganate, to provide the tinned surface with a protective oxide film has.

- weight) caustic soda are used.

been used commercially to prevent the appearance of such dark discolorations.

conjunction with the tinning operation, presents a number of difficulties. It is necessary to employ a 5 per cent. permanganate solution at or near 200 degrees Fahrenheit since the rate of the desired oxidation reaction is very slow at room temperature. fljl ie insoluble hydrated manganese oxide formed during the treatment has a tendency to stain the plate and, in order to obtain satisfactory results, a vigorous scrubbing of the plate, during the treatment, is necessary.

Further, the oxide film formed is not always of uniform thickness with the result that various areas of the tin plate sheet differ in appearance. To overcome the existing difficulties of the permanganate treatment, the present invention has been developed.

Under the present invention, the tinned material is made the anode in a solution of an electrolyte, preferably an alkaline electrolyte, through which an electric current is caused to pass. The surface of the tin coating is oxidized by the nascent oxygen generated by the electrolysis of the solution and a continuous oxide film is produced. In most cases, the treatment is continued for such time as is necessary to allow the film to become sufiiciently thick to appear pale yellow in color. Although the speed at which the oxide The permanganate treatment, however, when performed in Tin plate anodically treated, as above described, offers greater resistance to discoloration during the processing of. materials which normally discolor the tin. For example, corned beef is ordinarily processed at about 230 degrees Fahrenheit for approximately three hours and, by having the cans made from tin plate anodically treated under the present invention, the protective oxide film on the surface offers considerable resistance to the reaction which takes place during such processing and prevents discoloration of the surface of the tin plate.

The nascent oxygen necessary to the formation of the desired film results from the electrolysis of the water of the treating solution, while the electrolyte present in said solution serves merely to conduct electric current. Therefore, it becomes apparent that any one of a number of electrolytes may be used in preparing a solution suitable for the present treatment. The

' only limitation on the choice of the electrolyte "is that the solution prepared therefrom must not 35 "tinned surface.

etch or otherwise chemically attack the metallic Examples of electrolytes which have been used successfully in the present treatment are such alkaline compounds as caustic soda, sodiumflcarbonate, bo'rate, phosphate and silicate. These-materials may'be present in the solution individually or in combination. The concentration of electrolyte has been found to have little effect on the results obtained.

Maximum concentration limits, however, can be set from economic considerations. Since concentrated solutions are not only costly to prepare but also to operate, owing to the relatively large loss of electrolyte in the film of solution carried by the tinned material as it leaves the treating receptacle, it is impractical to exceed a concentration of 30 per cent. (by weight) of the electrolyte used. Further, in the specific case of caustic soda, a maximum concentration of 10 per cent. (by weight) can be set, since more concentrated solutions of this material are diflicult to rinse from the sufaces of the tin plate and will, if not immediately removed, have a deleterious effect on said surfaces.

While very dilute solutions of any of the aforementioned electrolytes may be used, we have found the use of intermediate concentrations to be preferable in order to minimize electrical power consumption. For example, the preferred concentration range of the electrolytes hereinabove set forth are: caustic soda 0.5 to 1.0 per cent. (by weight); sodium carbonate 5 to per cent. (byweight) sodium borate or phosphate 10'to per cent. (by weight), sodium silicate (meta or ortho) 0.5 to 2.0 per cent. (by weight). The above solutions may be operated at any temperature from room temperature to the boiling point of the particular solution used. The tinned material is subjected to electrolytic action as an anode in any one of the above solutions for a period of three to six seconds at a current density preferably about 'amperes per square foot.

When the concentration of caustic soda is greater than 1 per cent. (by weight), the temperature should not exceed 140 Fahrenheit, as it has been found that solutions containing more than 1 per cent. (by weight) of caustic soda will attack the tin surface if heated above this temperature.

Should a combination of the above electrolytes be desired, the solution through which the tinned material passes as an anode, may include an alkaline solution containing 150 grams of tri-sodium phosphate, 50 grams of sodium carbonate and 20 grams of caustic soda per liter of water, such proportions as hereinabove recited providing an electrolyte having good conductivity.

By treating the tinned material as hereinbefore described, the surface is provided with characteristic properties, which will not stain or discolor during the processing of materials which have a'tendency to react with tin.

tially the same conditions with respect to temper ature, time and current density as has been hereinabove set forth with respect to the anodic treatment. The electrochemical reactions taking place at the surface of the tinned material under the conditions of the cathodic treatment remove any initial oxide film present thereon. Other extraneous matter is carried away from the surface of the tinned material by the evolution of hydrogen.

By subjecting the tinned material as a cathode in the alkaline solution the undesirable film is removed to provide a clean surface upon the tinned material so that, upon reversing the direction of the current in the circuit within the bath, and making the tinned material the anode, the nascent oxygen generated within the bath at the anode, forms upon the tinned surface a uniform protective oxide film.

While this combined operation of cathodically treating the tinned material and anodically forming a uniform protective film on the surface of the cathodically treated material can be carried out in the same bath, separate baths may be The present anodic treatment of the tin plate is decidedly superior to the foregoing permanganate treatment because of its flexibility, simplicity, and economy of treatment. There are no interfering products formed in the oxidation reaction under the anodic treatment of the present invention, it is not necessary to heat the solution, the current density is readily varied to meet changing speeds of operation, no scrubbing of the tin plate is necessary, and a continuous protective fihn is formed on the tinned surface.

While the foregoing anodic treatment is far more satisfactory than the present day permanganate treatment, it does not always produce a film of substantially uniform thickness upon the surface of the tinned material. If any areas of the material are covered by oxide, prior to the I anodic treatment, the oxide on such areas is ultimately increased in thickness during the foregoing anodic treatment and, the resulting variation in thickness of the final oxide film provided upon the surfaces of the tinned material, sometimes causes a spotted appearance thereof.

In order to insure a more satisfactory appearance of he protective film provided upon the surface of the tinned material, we prefer to subject the tinned material to a cathodic treatment just prior to anodic treatment, in order to remove any initial oxide film or other extraneous matter present as aresult of previous processing .50 that there is provided a clean surface which will obtain the full benefit of the anodic treat- ;ample a solution of any one of the aforementioned compounds. The treatment of the tinned mateused to effect the same result, if such procedure seems desirable. While the reactions are satisfactory at approximately room temperature, the temperature of the electrolytic bath or baths may vary to meet specific conditions. It is to be understood that the current densities and time pe riod of the cathodic and anodic treatments may be varied to meet the variety of conditions encountered in practice.

It becomes apparent that under the present invention, the cathodic treatment of the tinned material to remove the undesirable film and other extraneous matter permits a substantially uni,- form, protective coating to be formed upon the surface of the material by a subsequent anodic treatment so that the tinned material Will not stain or discolor in subsequent steps during the packing of materials, which materials have a tendency to discolor or corrode the tin.

The present application is a continuation-inpart of the applications filed jointly by us on January 21, 1937, Serial No. 121,665, on 'Combined cathodic and anodic treatment of tin plate,"

and on January 21, 1937, Serial No. 121,666 on Anodic treatment of tin plate, and includes, as a single application, the inventions disclosed in the aforesaid applications.

While we have described specific embodiments of our invention, it will be understood that we do not wish to be limited exactly thereto, since various modifications may be made without departing from the scope of the invention, as defined by the following claim.

We claim:

The herein described method of forming a protective film upon tinned container stock, consistingin anodically treating said stock for not more than six seconds in an aqueous alkaline electrolyte at a current density sufiicient to cause nascent oxygen to be deposited on the tinned surface and thereby forming a continuous oxide film on said tinned surface. solely by said anodic treatment, said electrolyte being incapable of chemirial as the cathode is carried out under substancally attacking the tinned surface during said treatment, said film being of sufficient thickness that containers made from said stock are not discolored after or during the processing of sulphurbearing food-stuffs therein.

WILLARD O. COOK. HUGH E. ROMINE. 

